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Mobile home furnace (C) Daniel FriedmanCombustion Air Requirements
How to Recognize, Test, & Diagnose Heating Appliance Combustion Air Problems

Combustion air defects & hazards:

This article explains how to recognize and fix combustion air defects on heating appliances such as boilers, furnaces, and water heaters.

Lack of adequate combustion air causes improper heater operation, increased maintenance cost, and risks dangerous production of carbon monoxide gas. This article series explains how to recognize & diagnose problems with residential heating boilers, including loss of heat, heating boiler noises, leaks, odors, or smoke, and high heating costs.

This article series answers most questions about central hot water heating system troubleshooting, inspection, diagnosis, and repairs. Our photo at page top shows an oil fired furnace installed in a closet with an airtight door; there was no outside combustion air supply. The heating system could not work properly nor safely in this home.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Visible Signs of Inadequate Combustion Air in Buildings

Backpressure Burn at an oil burner (C) Daniel FriedmanHow to recognize, diagnose & fix inadequate combustion air, sooting, or burn marks at oil-fired or gas-fired heating systems.

Watch out: inadequate combustion air not only causes improper and wasteful operation of heating equipment it can also produce fatal carbon monoxide gas hazards indoors.

The quantity of combustion air needed depends on the fuel type, the input BTUh rating of the heating equipment, and additional air needed to assure effective exhaust draft to carry combustion products safely out of the building.

Topics discussed here include:

How to diagnose signs of inadequate combustion air for an oil or gas burner.

These photographs show a heating furnace with soot blow-back around the oil burner probably means there is backpressure in the combustion chamber - an improper operating condition that may be unsafe.

Backpressure sooting at an oil fired furnace (C) Daniel Friedman

Watch out: inadequate combustion air supply to a gas burner (and less often to an oil burner) is very dangerous and can produce potentially fatal carbon monoxide.

If you suspect unsafe heating system operation or a carbon monoxide problem be sure everyone leaves the building immediately and then call your local fire department for assistance.

Clues Indicating Possible Lack of Combustion Air & Related Safety Hazards

Mobile home furnace (C) Daniel Friedman

Lack of adequate combustion air can be indicated by or can result in these heating system operating and safety worries:

Also see UNSAFE HVAC DUCT OPENINGS which describes the risks of reduced combustion air on hot air heating systems when certain return air duct defects are present,
and
also see CARBON MONOXIDE - CO and CHIMNEY INSPECTION DIAGNOSIS REPAIR

Heating Equipment Combustion Air Rules of Thumb

Square Inches of Combustion Air Intake: 1 sq. in. per 1000 BTUH

For heating equipment installed in confined spaces, an old-timer's rule of thumb is to add up the total INPUT BTUH numbers from all of the data tags on all of the heating equipment installed. You want to see at least one sq. in. of open fresh air intake per 1000 btuh.

Total Sq .In. of Combustion Air Intake Opening = Total Input BTUH / 1000

Watch out: this combustion air rule of thumb needs to be adjusted to account for the air flow restriction caused by louvers and screening over the combustion air intake opening. As a rule of thumb we

Reduce the effective total square inches of combustion air intake opening by at least 1/3 for louvers & screens

Reduce the effective total square inches of combustion air opening further if the screen is or can easily become clogged by lint, leaves, trash & debris

Watch out: The combustion air estimate provided by outside combustion air openings or openings into other, presumably larger building areas (see below) also needs to account for the effects of building exhaust fans, tight buildings, and similar interferences.

See BACKDRAFTING HEATING EQUIPMENT.

Watch out: this rule of thumb falls apart if the fresh air is not being vented directly into the heating equipment area through an outside wall.

That is, if air has to move through vent pipes or ductwork into the area where it is needed, the equivalent square inches of fresh air intake venting may need to be increased depending on the length, number of bends, angles of bends, and diameter of the fresh air or combustion air intake venting system.

Standard engineering approaches to calculating air flow through round or rectangular ductwork can solve the question of impact on combustion air of routing it through ducts.

Cubic Feet of Room Space as a Measure of Adequacy of Combustion Air: Total Input BTUH / 1000 x 50

For heating equipment installed in larger spaces, a common rule of thumb for computing the required total cubic feet of free space to assure adequate combustion air is to provide 50 cubic feet of free space per 1000 Input BTUH for the total of all of the heating appliances installed in the area.

The assumption behind this old rule is that buildings leak air and that larger rooms or spaces have more air intake leaks than smaller ones.

Total Cubic Feet of Free Area = Total Input BTUH / 1000 x 50

Watch out: this formula may not adequately consider the reduction in volume of the room or open space attributed to contents, storage, etc. and it certainly does not adjust for modern tight building construction.

COMBUSTION AIR for TIGHT BUILDINGS explains how to provide outside combustion air for tight buildings.

Some writers simplify the formula to express this rule of thumb as

Total Cubic Feet of Free Area = Total Input BTUH x .05

Example: if we have a 180,000 Input BTUH boiler and a 40,000 input BTUH water heater installed in an enclosed utility room, how many cubic feet of space in that room would make us think we had adequate combustion air?

(180,000 + 40,000) / 1000 x 50

220,000 BTUH / 1000 = 220

220 x 50 = 11,000 cubic feet.

If our room is smaller than 11,000 cubic feet we probably don't have adequate combustion air (unless an outside combustion air source is also provided).

Calculate the total cubic feet of space in a room by multiplying the room width x room length x room height

Example: if the heating equipment is installed in an open basement that is 40 ft. x 20 ft. x 8 ft. high, we have

Cubic Feet = 40 x 20 x 80

Cubic Feet = 6400 - this basement will probably not provide adequate combustion air for the example input BTUH total given above.

Special thanks to reader Joe who corrected a math error in the above, 14 Jan 2016

Table of Combustion Air Supply Duct Sizes vs BTUh

Watch out: square duct area is not equal to round duct area in air flow capacity. That's because air flowing through a square or rectangular duct (or chimney) does not flow uniformly - the area of the corners of the rectangle moves less air.

For you who left your calculator at home and left Pi in the refrigerator, here is the square inches of cross section opening size for common round duct diameters:

Table of Combustion Air Supply Duct Sizes vs. Input BTUh for Natural Draft Heating Appliances

Round Duct Diameter
in Inches
Duct Cross Section Square Inches
(Round Opening Size)
Approximate Total Input BTUH Supported at This Combustion Air Duct-Vent Opening Diameter2
6
28
84,800 2
7
38
115,000 2
8
50
151,000 2
9
64
191,000 2
10
79
236,000 2
12
113
339,000 2
14
462,000 2

Notes to the Table Above

1. This data is for round ducts and smooth metal duct sides; lengths of flex duct with ribbed or corrugated sides restricts air flow and will not provide as much equivalent air flow.

Reference: "Evaluating Duct Work, How to Evaluate Furnace Duct Work & Cure Short Cycling or Inadequate Ductwork Problems" Vermont Department for Children and Families, Office of Economic Opportunity, - dcf.vermont.gov/sites/dcf/files/pdf/oeo/WAPManual/AppendixI.pdf‎ retrieved 12/5/2013

2. Really? We need further research on these figures. they significantly exceed the 1000 BTUH per square inch of area. Citation: David Clark, Home Inspection Newsletter, retrieved 12/5/2013 http://turnkeyhomeinspectors.com/turnkeyhomeinspectors_004.htm

3. These numbers are for round opening sizes used to provide combustion air to heating equipment.

See SUPPLY DUCTS & REGISTERS for details about HVAC heating or cooling supply & return air duct sizing, air flow, and for matching HVAC air duct sizes to equipment BTUH rate or heating capacity.

How to Convert Round Opening Diameters (say an air duct cross section) to Opening Size Equivalent

If we are using smooth-walled round ducts to bring combustion air into the space where it is needed, and before considering the restrictions on air flow caused by duct bends and length (friction losses) we start by simply calculating the cross-sectional area of the duct:

Pi r2 = the area of a circle or the cross-sectional area of a round combustion air supply duct

Pi (also written as Π) = 3.1416 - a constant

r = the radius = half of the diameter of the circle

We can use any unit (cm, inches, feet) as long as we stick to the same unit through.

Example: a 6-inch diameter round air duct has a cross section (or area) of

Area = 3.1416 x (6 / 2)2 inches

Area = 3.1416 x (3)2 inches

Area = 3.1416 x (3 x 3)

Area = 28 sq. in. of space - which, if unrestricted by duct length, bends, or screens, and if we use our first rule of thumb (one inch per 1000 BTUH) would support about 28,000 Input BTUH

Combine Combustion Air Sources to Check the Combustion Air Requirements

When room volume in cubic feet is inadequate to provide safe combustion air we can add combustion air by providing an outside air source.

Combustion Air Equipment Manuals & Codes

IRC Mechanical Code Section 71. COMBUSTION, VENTILATION AND DILUTION AIR Section M1701, G2407 (304) IRC 2009

Section M1701 General

M1701.1 Scope.

Solid-fuel-burning appliances shall be provided with combustion air in accordance with the appliance manufacturer’s installation instructions. Oil-fired appliances shall be provided with combustion air in accordance with

NFPA 31. The methods of providing combustion air in this chapter do not apply to fireplaces, fireplace stoves and directvent appliances.

The requirements for combustion and dilution air for gas-fired appliances shall be in accordance with Chapter 24.

M1701.2 Opening location.

In areas prone to flooding as established in Table R301.2(1), combustion air openings shall be located at or above the elevation required in Section R322.2.1 or R322.3.2.

G2407.1 (304.1) General.

Air for combustion, ventilation and dilution of flue gases for appliances installed in buildings shall be provided by application of one of the methods prescribed in Sections G2407.5 through G2407.9. Where the requirements of Section G2407.5 are not met, outdoor air shall be introduced in accordance with one of the methods prescribed in Sections G2407.6 through G2407.9.

Direct-vent appliances, gas appliances of other than natural draft design and vented gas appliances other than Category I shall be provided with combustion, ventilation and dilution air in accordance with the appliance manufacturer’s instructions.

Exception: Type 1 clothes dryers that are provided with makeup air in accordance with Section G2439.4.

G2407.5 (304.5) Indoor combustion air.

The required volume of indoor air shall be determined in accordance with Section G2407.5.1 or G2407.5.2, except that where the air infiltration rate is known to be less than 0.40 air changes per hour (ACH), SectionG2407.5.2 shall be used.

The total required volume shall be the sum of the required volume calculated for all appliances located within the space.

Rooms communicating directly with the space in which the appliances are installed through openings not furnished with doors, and through combustion air openings sized and located in accordance with Section G2407.5.3, are considered to be part of the required volume.

G2407.5.3 (304.5.3) Indoor opening size and location.

Openings used to connect indoor spaces shall be sized and located in accordance with Sections G2407.5.3.1 and G2407.5.3.2 (see Figure G2407.5.3).

G2407.5.3.1 (304.5.3.1) Combining spaces on the same story.

Each opening shall have a minimum free area of 1 square inch per 1,000 Btu/h (2,200 mm2/kW) of the total input rating of all appliances in the space, but not less than 100 square inches (0.06 m2).

One opening shall commence within 12 inches (305mm) of the top and one opening shall commence within 12 inches (305 mm) of the bottom of the enclosure.

The minimum dimension of air openings shall be not less than 3 inches (76 mm).

G2407.5.3.2 (304.5.3.2) Combining spaces in different stories.

The volumes of spaces in different stories shall be considered as communicating spaces where such spaces are connected by one or more openings in doors or floors having a total minimum free area of 2 square inches per 1,000 Btu/h (4402mm2/kW) of total input rating of all appliances.

G2407.6 (304.6) Outdoor combustion air.

Outdoor combustion air shall be provided through opening(s) to the outdoors in accordance with Section G2407.6.1 or G2407.6.2. The minimum dimension of air openings shall be not less than 3 inches (76 mm).

G2407.6.1 (304.6.1) Two-permanent-openings method.

Two permanent openings, one commencing within 12 inches (305 mm) of the top and one commencing within 12 inches (305 mm) of the bottom of the enclosure, shall be provided. The openings shall communicate directly, or by ducts, with the outdoors or spaces that freely communicate with the outdoors.

Where directly communicating with the outdoors, or where communicating with the outdoors through vertical ducts, each opening shall have a minimum free area of 1 square inch per 4,000 Btu/h (550mm2/kW) of total input rating of all appliances in the enclosure [see Figures G2407.6.1(1) and G2407.6.1(2)].

Where communicating with the outdoors through horizontal ducts, each opening shall have a minimum free area of not less than 1 square inch per 2,000 Btu/h (1,100 mm2/kW) of total input rating of all appliances in the enclosure [see Figure G2407.6.1(3)].

G2407.6.2 (304.6.2) One-permanent-opening method.

One permanent opening, commencing within 12 inches (305 mm) of the top of the enclosure, shall be provided.

The appliance shall have clearances of at least 1 inch (25 mm) from the sides and back and 6 inches (152 mm) from the front of the appliance.

The opening shall directly communicate with the outdoors or through a vertical or horizontal duct to the outdoors, or spaces that freely communicate with the outdoors (see Figure G2407.6.2) and shall have a minimum free area of 1 square inch per 3,000 Btu/h (734 mm2/kW) of the total input rating of all appliances located in the enclosure and not less than the sum of the areas of all vent connectors in the space.

Original source:

 




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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2020-12-15 - by (mod) -

Bending a powered and not powered draft heating Appliance into the same chimney sounds dangerous to me. There's at least a risk of backdrafting.

On 2020-12-14 by Robert Recher

I have a mechanical room that has two large furnaces and one water heater. The two furnaces do not have powered exhaust vents, the water heater does. Question: May these three appliances vent into the same chimney?

On 2018-02-24 - by (mod) -

MIguel

The situation you describe sounds unsafe - it could kill you with carbon monoxide poisoning.

Do not go to sleep in such a building. BE SURE you have properly located, installed, and tested carbon dioxide detectors (CO Monitors) in your home,

NOW go outside, call your heating company to inspect and repair the system.

On 2018-02-24 by Miguel

If outside air is flowing in ur heater closet and is not letting the heater furnes work and I'm getting bad headaches

On 2017-10-09 - by (mod) -

Frank,

The equation for complete combustion of propane is

C3H8 + 5 O2 → 3 CO2 + 4 H2O + heat

We use larger numbers to answer your question to avoid many decimal points in a very small number:

Burning 1 million BTUs of propane will produce 62.7 kg or 138 pounds of CO2.

Burning propane produces 0.98 gallons of water per 100,000 BTUH. That is, a 100,000 BTUH propane heater produces about a gallon of water per hour or about a gallon of water per 100,000 BTUs of heat produced by burning propane.

Propane combustion produces other substances as the US EPA notes:

LPG is considered a "clean" fuel because it does not produce visible emissions. However,
gaseous pollutants such as nitrogen oxides (NOx), carbon monoxide (CO), and organic compounds are
produced as are small amounts of sulfur dioxide (SO2) and particulate matter (PM).

Since at NATURAL GAS & PROPANE COMBUSTION PRODUCTS https://inspectapedia.com/hazmat/Gas_Combustion_Products.php we note that burning a gallon of propane produces 91,000 BTUS of energy (heat) and 1.64 pounds of water, we can figure that burning 1000 BTUs of propane will produce 1.64/91 gallons of water or 0.18 gallons of water. That water is of course in vapor form whose vapor volume will vary depending on temperature and perhaps other factors.

There we also note the other combustion products and volumes from burning propane. We avoid giving an answer in cubic feet (you didn't give me a cubic "what" measurement) because the volume of a gas depends also on its pressure (we can assume atmospheric pressure) and temperature (that we don't know for your heater but a typical gas furnace will be operating around 110-140 degF depending on when and exactly where you make the measurement).

On 2017-10-08 by Frank

Can someone tell me the cubic of gases (including water vapor) generated from burning 1000 btus of propane?

 

Combustion Air Requirements Specifications for Power Burners

For combustion air requirements for power burner fired heating appliances

see COMBUSTION AIR for POWER BURNERS

Combustion Air Inlet by Automatically Operated Louvers or Dampers

This topic has moved to a new article at COMBUSTION AIR INLET AUTOMATIC LOUVERS

Combustion Air Inlets vs Cold Drafts

This sub-topic has moved to COMBUSTION AIR INLETS vs COLD DRAFTS found

in COMBUSTION AIR INLET AUTOMATIC LOUVERS

Guide to a Simple Combustion Air Safety Check for Gas Fired Heating Appliances

See COMBUSTION AIR SAFETY CHECK

Combustion Air Defects & Safety Hazards at Mobile Home Heating System

We moved this article to COMBUSTION AIR SAFETY in MOBILE HOMES

Combustion Air for Woodstoves - How is It Provided?

Please see the new home for this article topic now found at COMBUSTION AIR for WOODSTOVES

A Guide to Combustion-Air-Related Heating Equipment Malfunctions & Their Implications

This topic moved to COMBUSTION AIR DEFECT EFFECTS


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